Barney B. Clark, the recipient of the first permanent mechanical heart, regained consciousness today, nodded his head, recognized his wife and indicated to doctors that he wasn't in pain.

"I think it's a success," said a tired but smiling Dr. William DeVries in a briefing after the seven-hour emergency transplant operation ended at dawn.

Describing the implantation of the plastic, cloth and metal heart as "almost a spiritual experience," DeVries said: "It isn't over yet, it's just a beginning." Clark, a retired dentist who at 61 was too old to be a candidate for a scarce human heart, was in critical condition today in the University of Utah Medical Center intensive-care unit. But his vital signs were stable. On Wednesday night he was so near death that his operation was conducted nine hours earlier than planned. Clark will spend the rest of his life connected by 6-foot hoses to 375 pounds of equipment mounted on a wheeled cart and plugged into an electrical outlet with a backup power source.

He was selected for the mechanical heart--which costs approximately $16,500 and runs on about $20 in electricity per month--because his condition was otherwise inoperable, and he had a "stable home situation" with a "reliable spouse" as well as "stable psychological mechanisms," a hospital spokesman said. Clark was suffering from cardiomyopathy, a deterioration of the heart that kills about 10,000 Americans a year.

The operation by the 38-year-old DeVries, the only physician authorized by the Food and Drug Administration to conduct the procedure, was the culmination of 25 years of research into an artificial heart.

The device is made of polyurethane, Dacron, Velcro, metal and graphite. Developed here by Dr. Robert Jarvik, it is remarkably simple in design. It consists of two main sections, slightly larger than the normal human heart's lower chambers, that can take over the pumping of blood through the body.

The only heart developed so far, it is too big for a woman or a small man. Clark is 6 feet 2 and weighs about 200 pounds.

The bulkiness of this pioneering device, which confines a patient to home, is expected to be supplanted in the future by a more portable unit that would offer greater freedom of movement. The device uses compressed air, and carries a three-hour backup air supply.

The operation was the first attempt to permanently implant an artificial heart. Texas heart surgeon Denton Cooley has twice inserted artificial hearts into patients but only as a temporary measure until a human heart transplant could be found. Both Cooley's patients died following the transplants.

An exhausted Jarvik, still dressed in the green scrub suit he wore during the operation, said shortly after it ended that he was "happy the patient is doing as well as he is now. . . . I did have fears it could have been much worse." He said he would consider the procedure a success when "I hear the patient say it was worthwhile to go through it."

Dr. Willem Kolff, one of the pioneers of artificial organ development in this country and the head of the University of Utah's artificial organs division, said the government's National Institutes of Health "has spent $7.5 million to support the research that made it possible to do the first human implantation."

The mechanical heart has been tested on cadavers and animals. One of the artificial hearts is still beating after 4 1/2 years in the laboratory.

Advocates of further development of the device believe it could meet a need not being met by human heart transplants, for which only relatively young people are considered due to the scarcity of hearts for transplant. The advocates say that in the future an estimated 50,000 Americans could be helped by the artificial device, considered much more risky.

Under FDA guidelines, only two types of patients may be considered for mechanical hearts: those with serious, inoperative heart disease with no other treatment options, and patients undergoing open heart surgery who are not expected to survive the operation.

Hospital spokesmen said that Clark began to consider the transplant operation about two months ago. His condition worsened within the last month, restricting him to bed, and he notified the hospital last week that he wanted to go ahead.

Clark was required to sign a formidable and extremely pessimistic informed-consent form. It stated that "there is no guarantee that the implantation of an artificial heart will add any additional time to my life expectancy. In fact, this implantation might shorten my life."

He was chosen after receiving unanimous approval from a six-member university committee consisting of DeVries, two other heart specialists, a social worker, a psychiatrist and a nurse.

The operation began unexpectedly late Wednesday when DeVries and his team of 15 physicians became concerned that Clark's condition was deteriorating so rapidly that he might die before the scheduled Thursday morning surgery. His heart was beating irregularly, a dangerous condition called cardiac arrhythmia.

Both Clark and his wife were eager to proceed, said DeVries, and at 10:15 p.m. local time (12:15 a.m. in Washington) he was wheeled into surgery for final preparations. An hour later, a long incision opened Clark's chest from sternum to abdomen. Shortly after midnight here, officials announced that most of Clark's heart had been removed to make way for the artificial organ. He was sustained by a heart-lung machine during the operation.

Surgeons left intact the two small upper chambers of his heart, the atria. Two small cups were sewn to the atria, which are continuing to receive used blood, and attachment tubes were sewn to the two main arteries that carry blood from the heart, the aorta and the pulmonary artery. Then the two chambers of the device were snapped into place.

Complications developed, and for several hours early this morning no news announcements were made. Finally, the hospital's vice president for health sciences, Dr. Chase Peterson, reported that the patient was "doing well."